In recent years, photothermographic materials, which provides no effluent resulting from wet type processing, have been strongly desired in the medical and printing fields with respect to environmental protection and workability, and, in particular, photothermographic materials for use in photographic techniques which can form clear black images at high resolution by heat development have been brought into the market and rapidly prevailing. Since these photothermographic materials are generally developed at a temperature of not lower than 80° C., they are called as photothermographic materials to be distinguished from conventional light-sensitive materials which are developed with a liquid developer at a temperature of 25–45° C.
Thermally developable light-sensitive materials of this type are conventionally constituted of a light-sensitive layer which contains high sensitive silver halide grains spectrally sensitized with dyes, organic silver salts and reducing agents; an anti-irradiation layer (AI layer) to prevent light scattering of light, which is irradiated on said light-sensitive layer, passed through said layer without having been absorbed and reflected randomly at the boundary of a support, an intermediate layer and an adhesive layer; or a back-coating layer (BC layer) provided on the opposite side of the support; as well as are provided with a protective layer on the light-sensitive layer and BC layer to prevent abrasion marks during handling.
Thermally developable light-sensitive materials are generally characterized by simple processing because images are formed only by heat development after exposure, however, it is important to improve image lasting quality after development because of no fixing process being provided. To improve the image lasting quality, it is preferable to develop at a high temperature to form images, however, fogging is liable to be generated and the sensitivity is decreased when the developing temperature is too high. Therefore, they are generally developed at the vicinity of 120±10° C.
As an anti-fogging agent, to utilize specific propenenitrile compounds is disclosed in patent literature 1, and to utilize specific alkene compounds is disclosed in patent literatures 2 and 3.
Further, as an anti-fogging agent or an image lasting quality improving agent, to utilize specific blocked nitrogen-containing heterocyclic derivatives are disclosed in patent literatures 4–7.
However, these compounds were still insufficient with respect to the image lasting quality after heat development.
Further, as a leuco dye, to utilize specific bisphenol compounds is disclosed in patent literature 8, and to utilize specific coupler leuco dyes is disclosed in patent literature 9, however, these compounds were still insufficient with respect to the image lasting quality after heat development.
Herein, the image lasting quality after heat development refers to heat resistance (prevention of fog increase) and light fastness (prevention of the maximum density decrease and silver tone change) after heat development.
Polyhalomethane compounds have been proposed as an anti-fogging agent for thermally developable materials because they can decrease fog by releasing a halogen radial by heat or light excitation. For example, U.S. Pat. Nos. 3,874,946, 4,452,885, 4,546,075, 4,756,999 and 5,340,712; Japanese Patent Publication No. 54-165; JP-A Nos. 50-137126, 7-2781 and 9-265150 (hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection); and Japanese Patent Publication No. 2-32614 are listed.
There are proposed that variety of chemical structures as groups to be bonded to the polyhalomethane group to enhance the releasing efficiency of a halogen, however, it cannot be said that sufficient ability has been achieved. That is, because sensitivity is often decreased when the releasing efficiency of a halogen radical is high, and fog is hardly depressed when the releasing efficiency of a halogen radical is low, resulting in difficulty of improving image lasting quality.
[Patent Literature 1] Japanese Translated PCT Patent Publication No. 2000-515995
[Patent Literature 2] JP-A No. 2002-207273
[Patent Literature 3] JP-A No. 2003-140298
[Patent Literature 4] JP-A No. 2000-330235
[Patent Literature 5] JP-A No. 2002-236335
[Patent Literature 6] JP-A No. 2002-258442
[Patent Literature 7] JP-A No. 2003-5323
[Patent Literature 8] JP-A No. 2002-169249
[Patent Literature 9] JP-A No. 11-231460